This invention relates to a process for making polyurethane prepolymers. In another aspect it relates to a method of removing unreacted isocyanate from urethane prepolymer. In still another aspect it relates to a method of preparing a urethane prepolymer using a stoichiometric excess of polyisocyanate but obtaining low amounts of oligomers and free diisocyanate in the product.
Urethane polymers such as polyurethane and polyurethane/urea elastomer form a very significant part of the synthetic polymer industry worldwide. In the formation of such products it is common to prepare a prepolymer by reacting a polyisocyanate with a polyol, generally in a ratio of two molecules of isocyanate to one mole of diol. In preparing the product for its ultimate use, this prepolymer is reacted with a chain extending agent, such as a short chain diol or aromatic diamine to produce the final product in the form of a polyurethane or urethane/urea polymer. These urethane polymers are useful in a variety of applications such as in protective coatings and molded articles.
The prepolymer made up predominantly of the 2:1 isocyanate/diol combination is the most desirable product but it is difficult to obtain in high yields. One reason for this problem is the occurrence of competing side reactions that join these monomers in different ratios, such as 3:2, 4:3, or 5:4 molecules of isocyanate to molecules of diol. In order to reduce the tendency of the process to form such oligomers, operators have used an excess of the isocyanate beyond the 2:1 stoichiometric requirement. If this is done, however, the prepolymer product contains unreacted isocyanate that must be removed without harm to the prepolymer itself. Free isocyanate in the product adversely affects the stability of the product and presents health problems because of the toxicity of the free isocyanate. Attempts to solve this problem have been the subject of considerable industrial research, as is apparent from the following cited references.
As early as 1980 a Japanese Patent Application of Matsui et al., No. S55-29083 was opened to public inspection disclosing the use of certain aliphatic hydrocarbon solvents, such as n-heptane, n-hexane, n-octane, 3-methylpentane, and 3-ethylpentane alone or mixed with aromatic hydrocarbons such as benzene and toluene to extract unreacted isocyanate from a prepolymer made by reacting .omega., .omega.'-diisocyanate dimethylcyclohexane with a low molecular weight polyol in a ratio of NCO/OH in the range of 2 to 10, preferably 5 to 8. It is said that the prepolymer formation can be carried out either without a solvent of in the presence of a solvent such as ethyl acetate, tetrahydrofuran, dimethylformamide, methylethylketone, and dioxane. The low molecular weight polyols disclosed all have very low molecular weights under 250 g/mol, such as 1,3-butylene glycol, 2-methyl-2,4-hexane diol, trimethylol propane, glycerin, trimethylol butane, 1,2,6-trimethylol hexane, pentaerythritol, and sorbitol. The stated object is to convert such prepolymers into films having good weather resistance.
U.S. Pat. No. 4,288,577, McShane, Jr. (1981) is directed to curing agents for urethane prepolymers but gives an example of preparing a prepolymer from methylenebis(4-phenyl isocyanate) (MDI) and polytetramethylene ether glycol (PTMEG) having a molecular weight of 1000. The mole ratio of isocyanate to diol was only 3:1 which is insufficient to insure that oligomers will not form. It is said that the product was extracted with hexane until essentially no isocyanate was found in the last extract. The extraction technique is not described, nor is it stated how much free isocyanate remained in the prepolymer.
A prepolymer process which deals more directly with the avoidance of oligomers is disclosed by Milligan et al. in U.S. Pat. No. 4,683,279, (1987). In this process short chain diols are condensed with a mixture of 2,4- and 2,6-toluenediisocyanates, preferably in isocyanate to diol mole ratios of 4:1 to 30:1. The reaction can be carried out in a suitable solvent such as acetone, dioxane, THF, and the like, and oligomer formation is said to be minimized. The unreacted isocyanate is removed by distillation, for example in a wiped film evaporation apparatus. We have found that wiped film evaporation (WFE) technology works well with low melting prepolymers such as those of Milligan et al., but is difficult to use with highly viscous polymers or with prepolymers having lower volatility isocyanates. A similar disclosure appears in U.S. Pat. No. 4,786,703, Starner et al. (1988) where WFE is recommended for removal of TDI to a level in the prepolymer of less than 0.15 weight percent. The prepolymer is essentially a 2:1 isocyanate/diol adduct.
Various other solutions to the problem of unreacted isocyanate (NCO) in such prepolymers have been suggested, as in U.S. Pat. No. 4,871,828, Blind et al. (1989) which proposes extracting the isocyanate with an inert gas such as CO.sub.2 in a liquid or supercritical state. These inventors recognized that prior art extractions had used hexane or octane as the extractant, but stated "extractions with a solvent for the diisocyanate but which is a nonsolvent for the condensate (such as hexane, octane, etc.) is lengthy and cumbersome. In fact, as soon as the nonsolvent is added, the condensate tends to precipitate in the form of a sticky mass from which the free diisocyanate monomer is difficult to extract completely." U.S. Pat. No. 5,051,152, Siuta et al. (1991) combines wiped film evaporation with use of an inert sweeping gas to remove unreacted isocyanate. An improvement on this technique is disclosed by Starner et al. in U.S. Pat. No. 5,202,001 (1993) whereby the inert gas is passed through a quantity of prepolymer which has passed through the evaporation zone and then the gas is passed through the evaporation zone in a flow countercurrent to that of the polymer.
The seriousness with which this problem is viewed is illustrated by U.S. Pat. No. 5,703,193, Rosenberg et al. (1997) which describes a complicated distillation of the prepolymer in the presence of two solvents. One of the solvents must have a boiling point below that of the diisocyanate and the other solvent must boil above the diisocyanate. Distillation is carried out in an agitated thin film evaporator or similar apparatus. Suitable lower boiling solvents include expensive compounds as trichlorobenzenes, methyl or ethyl adipates, N-methylpyrollidone, and dibutylgluterate. Suitable higher boiling solvents include dimethylphthalate, dibutyladipate and Sulfolane.RTM. of Phillips Petroleum Co. These solvents can be present during the prepolymer synthesis in amounts ranging from 5 to 85 weight percent of the reaction mixture. It is clear from the foregoing references that a simple, inexpensive method of removing residual isocyanate from urethane prepolymer is very much needed.